This invention relates to exciters for applying a force to a structure undergoing mechanical testing in order to study the mechanical response characteristics, modes of vibration or fatigue life of the structure. More particularly, the present invention is directed to an exciter for applying a dynamic torsional force to a rotating structure, such as to the shaft of an electrical generator or motor, the propeller shaft of a ship, or the like, during rotation.
Torsional exciters are known for testing a rotating structure under full speed and full load. One type of machine is shown in U.S. Pat. Nos. 2,157,903 and 3,112,643 and includes two lines of shafting geared together at opposite ends. The structure to be tested is interposed in one line of shafting. A motor or the like simultaneously drives the two lines of shafting. The lines of shafting are interconnected by means of a torque coupling which acts through one line of shafting and reacts against the other line of shafting so as to apply a torque to the structure to be tested. Such machines, however, have several disadvantages. The variety of structures which can be tested is limited by the physical dimensions of the lines of shafting, and, therefore, the machines do not have universal application. Furthermore, the torque couplings included in the machines are relatively complex and expensive to construct as well as difficult to adjust for applying a desired torque to the structure to be tested.
Another type of machine is disclosed by U.S. Pat. No. 2,384,987 and includes a circuit for energizing an electrical motor which rotates a structure to be tested wherein the circuit also superimposes an oscillatory signal for energizing the motor for producing a dynamic torsional force on the rotating structure. U.S. Pat. No. 3,495,447 discloses the use of two motors for mechanical testing, one energized for rotating a structure to be tested and the other connected in series with the first motor and the structure to be tested and energized for applying torque to the rotating structure. U.S. Pat. No. 3,693,402 discloses an electromagnetic exciter including a disc rotatable with a structure to be tested. The disc includes a plurality of radially extending members of magnetically permeable material. A magnetic field is generated through which the disc rotates. The field initially attracts the members to apply a positive torque and then tends to retain the members to apply a negative torque. Consequently, a vibrational torque is applied to the rotating structure. One disadvantage is that these machines are openloop. As a result, the torque applied to the rotating structure is not precisely controlled.
Another type of machine is disclosed by U.S. Pat. No. 2,452,031 and includes a motor for rotating a flywheel as well as a structure to be tested. The flywheel includes sector-shaped openings in which arms are mounted for pivotal movement. The arms are centered by means of compression springs within the sector-shaped openings. The arms are moved about the center position by a hydraulic device for producing a dynamic torsional force on the rotating structure. The frequency of torsional oscillation may be adjusted by varying the speed of a hydraulic pump motor, and the amplitude of torsional oscillation may be varied by adjusting the stroke of the pump. However, the machine is open-loop which is disadvantageous because the dynamic torsional force applied to the rotating structure cannot be controlled with a high degree of precision. Furthermore, adjustment of the speed of the hydraulic pump motor and the stroke of the pump for setting the desired frequency and amplitude of the dynamic torsional force is cumbersome. Moreover, conventional pump motors normally operate at 1800 RPM which translates into a maximum excitation frequency of 30 Hz. Consequently, the machine disclosed by U.S. Pat. No. 2,452,031 cannot be employed for mechanically testing turbine generators, for example, where 60 Hz., 120 Hz., 180 Hz. and other harmonic excitation frequencies are required.
One objective of the present invention is to provide an exciter for applying a dynamic torsional force to any size or shape of rotating structure.
Another objective is to provide a torsional exciter which includes many readily available off-the-shelf components, is easy to construct and is relatively inexpensive to manufacture.
An additional objective is to provide an exciter of the above type which can be easily adjusted for producing an dynamic torsional force having a desired amplitude and frequency.
Another objective is to provide an exciter of the above type with the capability for applying a dynamic torsional force having a frequency as high as 1000 Hz.
A further objective is to provide an exciter of the above type which is closed-loop so that the dynamic torsional force applied to the rotating structure can be precisely controlled.